The present invention relates to systems for monitoring a parameter at remote, inaccessible locations, and particularly to such systems which detect cracks in helicopter rotor blades. Even more particularly, the invention relates to such systems having the capability of indicating to a second location (e.g., a cockpit) not only the fact of a sensed condition (e.g., such as a blade crack) but also the specific location of the sensed condition (e.g., a crack).
There are many applications wherein it is essential that a selected parameter at a remote location be continuously monitored and a warning provided whenever the monitored parameter varies above or below a predetermined limit. One example is helicopter rotor blades which are subject to cracks or other mechanical flaws that threaten the structural integrity of the blades. Such cracks and other flaws begin in small localized areas of the rotor blades, and are usually not immediately visible to the naked eye. Thus, to avoid hazard to the helicopter crew and passengers, it is essential to detect such cracks and flaws at their outset.
There are known in the prior art essentially two types of helicopter rotor blade crack monitoring systems which have been developed in response to the above need. The first type of system is one which utilizes a combination pressure transducer/ radiation source on each of the helicopter blades along with a radiation detector mounted in an opposed position on the body of the helicopter. The blades are pressurized, and as long as the correct pressure is detected by the pressure transducer, no radiation signal is transmitted. Upon detection of blade pressure above or below a predetermined level, the transducer/radiation source transmits a signal to the radiation detector thereby triggering a warning indicator inside the cockpit. This type of system is represented by U.S. Pats. Nos. 3,985,318 and 3,739,376 to Dominey et al. and Keledy, respectively. As is evident from a review of Keledy, this type of system has been adapted to provide the cockpit with both an indication of the fact of a crack and the specific blade which is cracked.
The second type of blade crack monitoring system is the direct-wired system represented by U.S. Pat. No. 3,981,611 to Jensen, an embodiment of which is depicted in schematic form in FIG. 1 of the drawings herein. In this type of system, the hollow helicopter blades are similarly pressurized with dry nitrogen gas or some other gaseous product. As depicted in FIG. 1, the blades of the helicopter are fitted with pressure transducers/switches 1a-f, whose electrical contacts are kept in a normally open position by the gas pressure. Upon the occurrence of a crack in one of the rotor blades, the gas pressure in that blade varies, thus allowing the switch in that blade's particular pressure transducer/switch to close. Closing of the pressure switch provides a ground path for voltage source 2 through relay coil 3. Energization of relay coil 3 causes relay wiper 4 to close thereby lighting a warning lamp 5 in the helicopter's cockpit, alerting the helicopter crew to the fact of a faulty helicopter blade. In this type of system, electrical connections are made between the spinning rotor blade assembly and the stationary rotor mast via slip rings 6a-c.
As is evident, the latter type system has distinct advantages over the former type system in terms of simplicity of construction and overall cost effectiveness. However, unlike the radiation source/detector type system first discussed, the latter type system has not been adapted to provide to the cockpit crew an indication of both the fact of a blade crack and an indication of the specific blade which is cracked. Thus, until now, the latter system has been less desirable. The existing system of FIG. 1 could be modified to provide a separate circuit from each rotor blade to the cockpit, thereby providing the capability of isolating the faulty blade; however, this would require extensive additional wiring and slip rings, thus substantially increasing both cost and potential for mechanical and electrical failures.